Ultrapure water has been used as wash water for cleaning members in the field of electronic industry. The quality of ultrapure water used in semiconductor manufacturing plants and wafer manufacturing plants is strictly controlled. For example, resistivity (specific resistance): 18.2 MΩ·cm or higher, microparticles: the number of microparticles having a diameter of 50 nm or more is 100 or less and the number of microparticles having a diameter of 20 nm or more is 1,000 or less, microorganisms: 1 count/L or less, TOC (total organic carbon): 0.5 μg/L or less, total silicon: 0.05 μg/L or less, metals: 0.1 ng/L or less, and ions: 5 ng/L or less are required.
The following microparticles may remain inside an ultrapure water production system: microparticles such as dust contained in the air, silica microparticles, and aluminium microparticles that may enter the system during the construction (new construction, extension, or modification) or the maintenance of the ultrapure water production system; particles contained in water, such as remains of dead bacteria and iron rust; and microparticles derived from shavings of membranes, pipes, and the like which may be produced in the production process. There has been proposed a method in which cleaning is performed using an alkaline solution in order to discharge such microparticles to the outside of the system and reduce the content of microparticles having a diameter of 50 nm or more in ultrapure water to 1000 particle/L or less (Patent Literature 1).
Patent Literatures 2 and 3 describe a method for sterilizing an ultrapure water production system which enables the content of microorganisms in ultrapure water to be reduced.